Raised bumps on the skin of the arms may be caused by factors such as keratosis pilaris, eczema and urticaria. 1. Perifollicular keratosis: Perifollicular keratosis, also called hairy moss, the cause of which has not been fully clarified, is considered to be genetically related. The main manifestations are follicular papules, roughness of the hand, and lesions characterized by heavy winter and light summer, resulting in raised bumps on the arms. 2. Eczema: acute eczema caused by the rash is mainly dominated by herpes mound, with a tendency to ooze. Chronic rashes are mainly characterized by mossy changes, accompanied by obvious itchiness, easy to recur. If it occurs on the arm, it may lead to localized pimples and papules with itching. 3. Urticaria: allergic reactions caused by rashes are likely to occur in allergic people, such as contact, inhalation, ingestion of allergens, resulting in skin erythema, air masses, may appear on the skin raised bumps with obvious itching. In addition, flat warts, common warts and folliculitis may also cause raised bumps on the skin. It is advisable to consult a dermatologist at a hospital for a definitive diagnosis.